1. Field of the Invention
This invention relates to the field of automatic transmission control, especially to a test for instability in the operation of a torque converter bypass clutch.
2. Description of the Prior Art
An automatic transmission torque converter produces a hydrodynamic connection between an impeller wheel, driven by an internal combustion engine, and a turbine wheel, connected to the input shaft of the transmission. A bypass clutch alternately driveably connects the impeller and turbine by forcing surfaces of the clutch into friction engagement and releases clutch connection of the impeller and turbine so that the turbine is driven hydrodynamically by the impeller.
The bypass clutch or lock-up clutch can be operated by alternating its status among fully disengaged, fully engaged, and continuous slip mode wherein the rotational speed of the turbine is regulated in relation to the speed of the impeller by changing the magnitude of hydraulic pressure applied to the clutch. Hydraulic fluid is an actuating medium for establishing the operating condition of the clutch. The magnitude of the hydraulic pressure supplied to the clutch to produce the fully engaged condition is substantially higher than the pressure required to develop clutch torque capacity marginally sufficient to connect the turbine and impeller. To produce continuous slip operating condition of the bypass clutch, the hydraulic pressure supplied to the bypass clutch is modulated at moderate frequency so that the clutch friction surfaces continually engage and release. Operating a converter clutch with continuous relative motion can alter the operating characteristics of the friction surfaces of the clutch over a period of time, and can cause unstable operation of the clutch.
Softlock operation of the clutch improves driveability, reduces heat generation, improves fuel economy and reduces noise and vibrations. Converter clutch instability can be detected by the vehicle operation by its adverse effect on driveability.
Various techniques have been devised to monitor operation of a torque converter lock-up clutch and take corrective action to assure proper performance under certain conditions. For example, U.S. Pat. No. 4,428,259 describes a system for fail safe operation of a torque converter lock-up clutch. It provides for the clutch to be disengaged when neither a first nor a second control pressure is applied, and to be fully engaged when control pressure is applied and governor pressure is above a first magnitude. Application of the first or second control pressures result when governor pressure reaches a first magnitude and either a first actuating pressure is present or throttle pressure is lower than a specified magnitude.
U.S. Pat. No. 4,785,689 describes a fail safe system for controlling an automatic transmission that involves detecting an abnormal relationship between the output shaft speed of the transmission and either the engine speed or the speed of a component of the transmission. When the abnormality is present, the magnitude of hydraulic pressure in the control circuit is increased so that slippage of clutches and brakes in the transmission is eliminated.
U.S. Pat. No. 5,046,175 describes a system for producing a signal indicating a malfunction in the operation of an automatic transmission. After the control system produces a signal that would ordinarily cause an offgoing clutch to become fully exhausted and clutch slip has not occurred after the signal is produced the number of no slip clutch conditions that occur within a predetermined time are counted, compared to a predetermined number, and used to produce a failure signal when the count exceeds a predetermined number. Slip condition of the clutch is determined by monitoring and comparing turbine speed and the output speed of the transmission.